What Einstein (and so many teachers and textbooks before and after him) ignored is that the real reason that airplanes go up (“generate lift”) is simply that they push air down.
What Goes Up Always Pushes Down
The most important thing to keep in mind if you want to understand how airplanes fly is that the air around us acts just like a liquid. We breath this “liquid,” we walk through this “liquid,” and in a balloon we can float in this “liquid.” The only way to raise yourself when you’re treading water is by pushing water down. An airplane rises in much the same way: by pushing the air around it down.
There are airplanes that can take off vertically by directing the exhaust from their jet engines down toward the ground (called VTOLs, for Vertical Take Off and Landing aircraft). They are literally pushing air down, which results in the airplane going up. Most airplanes, however, use a different technique to move air down.
Airplane wings push air down in the same way that a rudder on a boat works: by deflecting the fluid that’s rushing by. Of course, a boat’s rudder moves water to the left or right whereas a wing moves air up or down, but the principle is the same. The trick is to point the trailing edge of the wing in the direction you want the air to go (usually down). As air flows over and under the wing, it gets deflected downward.
The Bernoulli effect says that when water is forced through a narrow area, the water’s speed increases and the pressure drops. Similarly, air speeds up when it travels over a wing, and as it speeds up the air molecules are spread more thinly over the wing. The thinner the air, the fewer molecules press against any one point on the wing. The fewer molecules pressing, the lower the pressure.
Air follows the shape of the wing (top). Increasing the angle of attack entrains the air farther down (bottom).
Sir Isaac Newton’s third law of physics states that for every action there is an equal and opposite action. So as the air moves down, the wings rise up and take the rest of the airplane with them.
How to Train the Air
If you stick your hand out of a car window while driving down the road, fingers raised at a slight angle against the oncoming wind, your hand will be pushed up because it is deflecting air down. In this case, the majority of the lift (the power that’s raising your hand) comes from the air deflected off your palm. It turns out that this is a relatively inefficient way to create lift—think about how fast the car needs to be moving before you feel the effect.
Strangely enough, most lift acting on an airplane wing doesn’t come from air deflected off the bottom but from air flowing over the wing’s top surface. In other words, airplanes don’t fly like kites or parasails; airplanes gain lift in a completely different way.
Remember that air is like a liquid, and liquid is slightly “sticky.” When you pour a fluid (like tea) out of a carafe too slowly, it sticks to the carafe and dribbles out onto the table, right? This tendency for liquid to follow the shape of an object is called the Coanda effect, and it explains why air moving over the top of the wing gets “pulled down” along the shape of the wing. Because the trailing edge of the wing is pointed slightly downward, the air passing over the wing flows in that same downward direction. The Coanda effect essentially helps the wing push air down.
This is nonintuitive, so try the following experiment: Use two fingers to hold a teaspoon by the end of its handle upside down over a sink. Now turn the faucet all the way on and gently bring the underside of the spoon against the rushing water. As soon as the spoon touches the water, the liquid sticks to the curved surface and shoots diagonally down off the spoon. The result: The spoon gets “pushed” in the opposite direction, into the water stream. If you turn your head sideways (and squint your eyes), you can sort of see the spoon as a wing getting pushed up into the air rushing by.
Those dimples on a golf ball actually trap the air and help it to better follow the surface of the ball. When a ball has backspin, the air traveling over the ball is entrained toward the ground, which helps the ball stay in flight longer, similar to an airplane wing (though wings are so big, they don’t need dimples).
Here’s where the Bernoulli principle kicks in. As the air is entrained along the top of the wing, it is “pulled” down and back, causing both a drop in pressure and an increase in speed. The lowest pressure occurs where the air is diverted the most: just behind the wing’s curved leading edge. The air is deflected downward, the pressure drops above the wing, and the aircraft rises.
Attacking the Air
Raising your fingers against the oncoming wind as you put your hand out a car window is called raising the angle of attack in aviation terms, and the more you raise the angle, the more lift you get (the faster your hand goes up). Similarly, when you raise a wing’s angle of attack (pointing the nose of the airplane up, for instance), you get more lift because the air is deflected downward faster. The trick to flying a plane upside down is in keeping the front edge of the wing higher than the back edge in order to deflect air down (toward the ground) and create lift.
However, there’s a problem: Air is sticky, but it’s not that sticky, so beyond a certain angle the Coanda effect stops working and the air won’t follow the top surface of the wing anymore. At this point—called stall—the air no longer gets deflected downward, and the wing can no longer generate lift. On most airplanes, stall begins when the angle of attack is greater than about fifteen degrees. (Note that the lift doesn’t just suddenly disappear; it’s a gradual process that can be reversed by dipping the nose of the aircraft down.)
In aviation lingo, the word stall has nothing to do with an unplanned loss of engine power, which is called an engine failure.
When fighter jets zoom up into the air at steep angles, their lift is based more on their powerful engines (which act almost like rockets) than on their wings. Commercial jet airliners and smaller aircraft, however, have to rise slowly in order to keep from stalling.
A Delicate Balance
Some people don’t like thinking about details, like how extraordinary it is that algae in the ocean account for much of the oxygen we breathe, or that thoughts are technically the result of brain cells passing chemicals to each other 1 million times a second. When we look too closely, we find that every moment of our lives is based on a delicate balance among mysterious forces that can be dimly grasped but hardly believed. Airplanes can fly because we humans have learned how to manipulate the air around us and hang in that balance.
Ignorance is the curse of God. Knowledge is the wing wherewith we fly to heaven.
—William Shakespeare, HENRY IV,
part 2, act 4, scene 7
When you hear that an airplane flies at 500 mph (800 km/hr), that’s the speed at which the air is traveling over the wings (airspeed), not necessarily how fast the airplane is moving over the ground (ground speed). An airplane flying into a headwind is like a boat traveling upstream—it takes longer to cover the same distance. For example, an airplane flying with an airspeed of 500 mph into a 50-mph headwind would have a ground speed of only 450 mph. But flying with a 50-mph tailwind boosts the ground speed to 550 mph, getting you to your destination faster.
What Is Air?
The only reason it is hard to understand how a fully loaded jet aircraft can fly is that air is invisible. If water were invisible, you might have a harder time believing that an ocean liner could float or a penguin could swim. And yet, air is as real and substantive as water. You can’t see the air, but you can see and feel its effects: grass moving in a gentle breeze, objects flying through the air in a hurricane, the steady erosion of rock along a windy beach.
Air is a relatively thick syrup of oxygen and nitrogen, along with some water vapor and a smattering of other elements. Like everything else, air is affected by gravity, so it has weight. However, the weight of air changes depending on a number of factors, including temperature (hot air is lighter than cold air) and humidity (believe it or not, the higher the humidity, the lighter the air, because the molecules in water v
apor are actually lighter than the weight of the gases in dry air). The weight also depends on air pressure, which is greater at sea level than it is at the top of a mountain because at higher altitudes there is less atmosphere “pushing down.”
The airplane stays up because it doesn’t have time to fall.
—Orville Wright
Airplanes can fly because they push the air around them down. A Boeing 737 weighing 150,000 pounds (68,000 kg) must deflect about 88,000 pounds (40,000 kg) of air—over a million cubic feet (31,500 cubit meters)—down by 55 feet (16.75 m) each second while in flight.
Technically, air is very light (one cubic foot of it at sea level weighs about 1.25 ounces, or thirty-five grams), but it actually exerts an incredible amount of pressure because there is so much of it. At sea level, every object touching the air (including your skin) experiences about 14.7 pounds of pressure per square inch (or about 1 kilogram per square centimeter). That’s like tons of weight pressing against your skin each moment! However, you can’t feel it because you have air pressure inside you, too. We’re all so used to this equilibrium that it is difficult to feel the air around us.
The man who flies an airplane…must believe in the unseen.
—Richard Bach author of
JONATHAN LIVINGSTON SEAGULL
Seeing Air
Although air is full of molecules, it’s invisible not just because the molecules are too small to see, but also because they don’t reflect or absorb the light waves that humans can see. On the other hand, you can see water vapor (like clouds), dust, or pollution particles suspended in the air.
For example, airplanes make clouds called contrails(condensation trails) at high altitudes when water vapor latches on to tiny particles in the engine exhaust. These long clouds can dissipate quickly or last for hours, depending on the humidity and the wind conditions.
Similarly, plumes of cloud can appear when an airplane flies in relatively humid air—that is, air carrying water vapor. Air traveling over the wings is moving faster and at a lower pressure than the air around it, and lower air pressure forces the water vapor to rapidly condense into clouds. These low-pressure plumes typically appear at takeoff or landing, either over the wings or at the tips of the wings. (You can also see these kinds of clouds as wind blows over tall buildings or mountains.)
You can also sometimes see the air where areas of cool and hot air meet (like the air above an asphalt road in the summer) because light travels slightly faster through the less dense hot air than it does through cool air. The result is a shimmering “mirage.” The air may fool the eye, but it’s as real as anything. So the next time you find yourself on an airplane wondering what’s between you and the ground, try imagining you’re a fish surrounded by clear water that you can’t see, but which is supporting you nevertheless.
One of the most joyous experiences when flying is breaking through the cloud layer into the blue sky above. The above-the-clouds perspective can be fascinating, especially if you know what to look for. Here’s a rundown of the basic types of clouds you might see, either from an airplane window or from the ground
LOW CLOUDS. The lowest clouds in the sky generally consist of water droplets. Stratus clouds are the lowest of the clouds and don’t have well-defined edges. (You can think of fog as stratus clouds at ground level.) The word stratus comes from the Latin “to stretch or extend.” Cumulus clouds are the opposite: Fluffy and popcornlike, these are the most stereotypical of clouds (the name comes from the Latin “to heap or pile”). Although cumulus clouds appear serene, they typically appear in more turbulent air than the truly calm stratus clouds. Another low-flying cloud is the cap cloud (sometimes also called a wave cloud), which appears over mountains or even some tall buildings.
MIDDLE CLOUDS. Clouds at 7,000 to 18,000 feet are thicker, consisting of ice crystals or water droplets. Altocumulus clouds look like a thick textured fabric, perhaps canvas, or a wool blanket. Altostratus clouds are more diffuse and uniform, like a blotchy cotton sheet.
HIGH CLOUDS. The highest clouds in the sky are composed of ice crystals and appear above 18,000 feet, sometimes even at airliner cruise altitudes (around 28,000-35,000 feet). From the ground you can see the sun or moon through these clouds (sometimes with halos around them). Cirrus clouds are thin, wispy, and often gently curved liked hairs in the sky; in fact cirrus is Latin for a “curl” or “fringe.” Cirrocumulus clouds are like a thin sheet of delicate textured fabric, like wavy cotton gauze. Cirrostratus clouds are more diffuse and blurrier, blanketing the sky like wax paper.
MULTILAYER CLOUDS. Thick and dark storm clouds are typically made of nimbostratus clouds, which may reach from the low to the middle strata. Cumulonimbus clouds, perhaps the most dramatic-looking of clouds, tower into the sky, sometimes as high as 60,000 feet, often with a characteristic anvil-shaped head. These clouds can produce lightning, thunder, hail, heavy rain, and extremely strong winds, and pilots go out of their way to fly around them.
Aviation is proof, that given the will, we have the capacity to achieve the impossible.
—Captain Edward “Eddie” Rickenbacker
Parts of an Airplane
Why can’t an airliner’s doors open during flight? Because they’re actually wider than the door frame itself. At the gate, the door first opens inward, then rotates, then slips sideways out of the frame. However, there’s no way to open a door when an airliner is flying at cruise altitude because the air inside a jet airliner is pressurized to about seven pounds per square inch and the ambient outside air at 35,000 feet is about 3.5 pounds per square inch. Even the smallest exit door covers about 650 square inches; opening one of these would be like lifting a 2,200-pound weight. Airline doors—also called plug doors—are designed so they can be opened even with a quarter-inch (7 mm) of ice covering the outside of the airplane.
Wings: The wings are cambered (somewhat curved on top and flat on the bottom) and are slightly higher at the tips (dihedral), giving the airplane additional stability and strength.
Winglets: An upturned wing tip is called a winglet. In some aircraft, winglets can increase efficiency (you can fly farther on less fuel) by reducing the amount of air that spirals up from underneath the wing. Blended winglets are extensions that curve smoothly up at a smaller angle than normal winglets.
Slats: At takeoff and landing, the pilots extend the slats at the front edge of the wing, effectively extending the wing so that it can fly better at slower speeds. These are usually found on larger jets.
Flaps: The pilots extend the flaps along the trailing edge of the wings at takeoff and landing for the same reasons birds spread their feathers: for increased lift at slow speeds. During flight, the pilots retract the flaps and slats for a more streamlined wing.
Spoilers: Flat panels along the top edges of the wings that, when raised, “spoil” some of the lift by creating drag. The pilots can raise the spoilers on both wings in flight to descend faster, or raise one wing’s spoilers to help turn the aircraft. Both spoilers pop up at full force as soon as the airplane lands to keep the airplane firmly on the runway. Some people also call spoilers speed brakes.
Ailerons: The pilots roll the airplane to the left or right by raising one aileron and lowering the other, which deflects the air up or down. (The ailerons typically move only a few inches, even on a jumbo jet.) The pilots turn the aircraft by adjusting both the ailerons and the rudder.
Vertical stabilizer: The part of the tail that sticks straight up and helps the airplane fly straight, like the tail feathers on an arrow. Some high-tech aircraft, like the stealth bomber, don’t have a tail. On a paper airplane, the vertical stabilizer usually descends from the bottom.
Rudder: The rudder on an airplane works just like a rudder on a boat: by deflecting the air to the left or the right. Turning the rudder to the left turns the tail to the right and the front of the airplane to the left, causing the aircraft to slip slightly sideways through the sky. The pilots normally turn the airplane through the sky by adjusti
ng both the rudder and the ailerons.
Horizontal stabilizers: A set of smaller wings on the tail that offset the lift of the primary wings just enough to balance the airplane so it doesn’t pitch up or down. The pilots usually adjust the angle of the horizontal stabilizer before takeoff based on the weight and balance of the airplane.
Elevators: Control surfaces that attach to the trailing edge of the horizontal stabilizers and, when adjusted, change the pitch or attitude of the airplane (causing the nose of the aircraft to move up or down). Just before takeoff, the elevators are pivoted up slightly, pushing the tail down and the nose up.
Trim tabs: Small surfaces at the trailing edges of the ailerons, horizontal stabilizer, and rudder that help balance the airplane. For instance, if the balance is slightly too far back and the nose of the airplane is tending to raise up, the pilots can adjust the trim tabs to counteract this tendency, bringing the airplane back into level flight.
Static wicks: The friction of air moving over the skin of the aircraft builds up a static electrical charge. Small extensions called static wicks look like antennas along the trailing edge of the wings and tail release that charge into the air.
The Flying Book Page 2
